Using two different elements creates new possibilities in hybrid atomic quantum computers

March 02, 2022

(News from Nanowerk) Qubits, the building blocks of quantum computers, can be made from many different technologies. One way to create a qubit is to trap a single neutral atom in place using a focused laser, a technique that won the Nobel Prize in 2018.

But to create a quantum computer from qubits of neutral atoms, many individual atoms must be trapped in place by many laser beams. Until now, these lattices have only been built from atoms of a single element, for fear that creating a lattice from two elements would be prohibitively complex.

But for the first time, researchers at the University of Chicago have created a hybrid lattice of neutral atoms from two different elements, greatly expanding the system’s potential applications in quantum technology. The results were funded in part by the NSF Quantum Leap Challenge Institute Hybrid Quantum Architectures and Networks (HQAN) and published in Physical examination X (“Two-element two-dimensional matrix of atoms with continuous mode operation”).

Left: A hybrid lattice of cesium atoms (yellow) and rubidium atoms (blue). Right: The researchers’ customization of the technique allows them to place the atoms anywhere, allowing them to create this image of Chicago’s Willis Tower and Cloud Gate landmarks. The scale bar in both images is 10 micrometers. (Photo: Hannes Bernien)

“There have been many examples of quantum technology that have taken a hybrid approach,” said Hannes Bernien, the project’s principal investigator and assistant professor at the University of Chicago’s Pritzker School of Molecular Engineering. “But they have not yet been developed for these neutral atom platforms. We are very happy to see that our results have triggered a very positive response from the community and that new protocols using our hybrid techniques are being developed.

Double the potential

While artificial qubits such as superconducting circuits require quality control to remain perfectly coherent, neutral atoms made from a single element all have exactly the same properties, making them ideal and coherent candidates for qubits.

But since every atom in the lattice has the same properties, it’s extremely difficult to measure a single atom without disturbing its neighbors – they’re all on the same frequency, so to speak.

“There have been quite a few landmark experiments over the past few years showing that atomic lattice platforms are extremely well suited for quantum simulation and also quantum computing,” Bernien said. “But measurements on these systems tend to be destructive, since all atoms have the same resonances. This new hybrid approach can be really useful in this case.

In a hybrid lattice composed of atoms of two different elements, the nearest neighbors of any atom can be atoms of the other element, with completely different frequencies. This makes it much easier for researchers to measure and manipulate a single atom without any interference from surrounding atoms.

It also allows researchers to avoid a standard complication of atomic networks: it is very difficult to keep an atom in one place for very long.

“When you do these experiments with single atoms, at some point you lose the atoms,” Bernien said. “And then you always have to reset your system by first creating a new cold cloud of atoms and waiting for the individual atoms to be trapped again by the lasers. But thanks to this hybrid design, we can experiment with these species separately. We can experiment with atoms of one element, while we update the other atoms, and then we change to always have qubits available.

Make a bigger quantum computer

The hybrid network created by Bernien’s group contains 512 lasers: 256 charged with cesium atoms and 256 with rubidium atoms. When it comes to quantum computers, that’s a lot of qubits: Google and IBM, whose quantum computers are made of superconducting circuits rather than trapped atoms, only reached about 130 qubits. Although Bernien’s device is not yet a quantum computer, quantum computers made from atomic lattices are much easier to scale, which could lead to important new information.

“We actually don’t know what happens when you scale a very coherent system that you can isolate very well from the environment,” Bernien said. “This trapped atom approach can be a great tool for exploring quantum effects of large systems in unknown regimes.”

The hybrid nature of this network also opens the door to many applications that would not be possible with a single species of atom. Since the two species are independently controllable, atoms of one element can be used as quantum memory while the other can be used to perform quantum computations, assuming the respective roles of RAM and CPU on a typical computer.

“Our work has already inspired theorists to think about new protocols, which is exactly what I was hoping for,” Bernien said. “I hope this gets people thinking about how these tools can be used for measurements and condition monitoring. We’ve already seen some really cool protocols that we’re very interested in implementing on these berries.”